Sandy has weakened to a category 1 hurricane due to strong vertical wind shear and dry air intrusion. Some further weakening is expected in the near term as the storm interacts with a nearby upper-level low. Sandy is currently moving slowly toward the north. An increase in forward speed along with a turn toward the NE are expected over the next 48 hours. Late this weekend, the storm will begin to interact with a trough that will be approaching the Eastern Seaboard. This will cause the storm to turn toward the NW and intensify as it undergoes extratropical transition. Hurricane Sandy will eventually merge with the approaching trough, which will cause its impacts to be felt across a very broad area from the Mid-Atlantic to the Northeast. Models are still divergent with regard to where the center of the storm will make landfall, with some projecting a landfall further south near the DelMarVa Peninsula while others are projecting a landfall further north toward New England. Regardless, tropical storm force winds will be very widespread with some areas likely to experience hurricane force winds.

The ICAT Damage Estimator (www.icatdamageestimator.com) can be used to obtain statistics regarding historic storms that have followed similar paths to Hurricane Sandy’s current forecast path. For this analysis, the Active Storms search feature was used to select all historic storms that have made landfall within the current range of computer model forecasts. This range includes the coastline from near the MD/VA border on the DelMarVa Peninsula to the eastern edge of Long Island, NY. The ICAT Damage Estimator shows that there have been 7 damaging tropical cyclones that have made landfall along this segment of coastline since 1900. The tool shows the storm parameters, the damage at the time of landfall, and the estimated damage if the storms were to make landfall in 2012. The 2012 damage estimations are made by “normalizing” the data by adjusting for population change, inflation, and change in wealth per capita.

The most damaging storm to make landfall within the current range of computer model forecasts was the New England hurricane of 1938, which would cause an estimated ~$47B in damage today. However, this storm was a category 3 hurricane when it made landfall, while Sandy is only expected to have category 1 force winds. Of the 7 storms selected, only two made landfall as category 1 hurricanes. Hurricane Agnes of 1972 made landfall with 85 mph sustained winds near New York City and would cause an estimated $19B in damage today. Agnes initially made landfall over the FL Panhandle, then moved NE and emerged off the NC coast. As it approached New England, the storm strengthened as it underwent extratropical transition, which is also expected to occur with Hurricane Sandy. Hurricane Agnes’s impacts were felt across a very wide area of the Northeast. Hurricane Belle of 1976 also had 85 mph winds, but moved much more quickly than Agnes and was weakening as it made landfall. It is estimated that Belle would cause less than $1B in damage today.

As can be seen from the storms selected by the ICAT Damage Estimator, the sample size of category 1 hurricanes making landfall along the Northeast is not very large. While Agnes appears to be the most similar to Sandy, it made landfall near New York City, which explains why the damage estimates are so high. Hurricane Irene of 2011 officially made landfall further south, but impacted a similar area that will be affected by Hurricane Sandy. That storm caused ~$7B in damage, but was not quite as strong as Sandy is expected to be. This data can be used as a benchmark to assess the range of possibilities for Sandy’s impact. The image below is output from the ICAT Damage Estimator, which shows the range of computer model forecasts and the historic storms that have made landfall within that range.Hurricane Agnes is highlighted in orange. The histogram shows normalized damage statistics for the 7 selected storms.

ICAT Damage Estimator Output

Visible Satellite Image

The ICAT Damage Estimator can be used to select and view historic storms that have similar characteristics to Isaac. For this analysis, the Active Storms search feature is used to select all storms that have made landfall within the current NHC forecast cone. The search results are further filtered using the All Storms search feature. The graphic below shows each historic storm that has made landfall within the current forecast cone as a category 1 or 2 hurricane.

The histogram on the left summarizes the normalized damage from these storms. It is estimated that 10 of the 15 would cause less than $1B in damage if they were to occur in 2012, while the other 5 would cause between $1B and $5B. The most similar historic track appears to be Gustav from 2008, which made landfall as a category 2 hurricane (track highlighted in orange). While Gustav was stronger than Isaac is expected to become, its center tracked well to the west of Baton Rouge and especially New Orleans. Isaac’s ultimate economic impact will be highly dependent on how closely it tracks to these population centers. The further west it tracks, the higher the likelihood it will fall within the lower range of the storms in this analysis, while the opposite is true the closer it tracks toward New Orleans and Baton Rouge. Regardless of the exact landfall location, tropical storm force winds will be felt across a very broad area due to Isaac’s large size. Tropical storm force winds currently extend outward up to 205 miles.

Model Track Forecast

Visible Satellite Image - 08/27/2012 - 21:15 UTC

As Irene approaches landfall, the ICAT Damage Estimator can be used to find historic landfalling storms that have followed a similar path. Using the Map Tools feature, a segment was selected on the Outer Banks of North Carolina from Cape Lookout to the NC/VA border. The ICAT Damage Estimator shows that 19 historical landfalling tropical cyclones have passed through this segment (pictured below). The most damaging of these was Storm 7 of 1944 (also known as the Great Atlantic Hurricane), which  brushed the Outer Banks with category 2 winds and made a final landfall over Long Island, NY with category 3 winds (highlighted in orange). This was the first storm to have a name designated by what is now known as the National Hurricane Center. The name was given to emphasize the storm’s size and strength as it reached category 4 status near the Bahamas. It is estimated that this storm would cause upwards of $19B in damage today when including both of its landfalls. The path of the 1944 Great Atlantic Hurricane appears very similar to Irene’s forecast track, though Irene is not expected to make landfall over the Northeast as a major hurricane.

Using the Active Storms tool, you can display the computer model forecast range for Irene and all historical storms that have made landfall within that range. The most damaging of these was the New England Hurricane of 1938, which made landfall over Long Island, NY as a category 3 hurricane and then moved inland over CT, MA, and NY (highlighted in orange). This is the 8th most damaging hurricane in the normalized record and would cause an estimated $46B in damage today. The multi-colored lines below show the current model forecast tracks. The legend at the bottom right shows which models they correspond to. The bold orange line originating in the central Atlantic represents the New England Hurricane of 1938.

Tropical Storm Don continues to feel the effects of moderate vertical wind shear and dry air entrainment. Sustained winds remain at ~45 mph, but the minimum central pressure has risen to 1005 mb. The storm has sped up slightly and is now moving briskly toward the NW at ~16 mph. Model guidance has shifted toward the south with regard to Don’s track. Accordingly, the NHC forecast track has shifted to the south as well, though it still remains north of most of the models. Don is not expected to reach hurricane strength prior to landfall due to the aforementioned inhibiting factors. The official NHC forecast calls for Don to make landfall late Friday as a moderate tropical storm with sustained winds of 55-60 mph.

The main story from Tropical Storm Don will likely be its rainfall. Much of southern Texas is currently in the midst of an “exceptional drought.” The Climate Prediction Center has most of Texas in the worst drought rating category (D4), with adverse agricultural and hydrological impacts felt widely.

The Hydrometeorological Prediction Center is predicting that Don will drop a substantial amount of rain on southern Texas over the next few days. Many areas that have not seen rainfall in a long time will be given some relief by Tropical Storm Don. Heavy rain on drought stricken areas can sometimes lead to flooding. With most areas expected to receive less than 4 inches of rain, significant flooding should not be a major issue with Tropical Storm Don.

The ICAT Damage Estimator can be used to compile a list of the most damaging tropical cyclones to make a U.S. landfall after today’s date (October 15th). Three of these late-season storms fall in the top 15 of the most damaging tropical cyclones on record (normalized to 2010). The most potentially destructive storm to make landfall after October 15th was Storm 11 of 1944. It formed in the western Caribbean and made landfall over southwestern Florida. After crossing the peninsula, it made a second landfall near Hilton Head, SC. It is estimated that these two landfalls would results in over $50B in damage today. A list of the top 5 most damaging late-season tropical cyclones is below. A map of their tracks was created using the Google Earth export feature in the ICAT Damage Estimator.

Tracks of the Top 5 Most Damaging Late-Season Tropical Cyclones

Hurricane Paula is the 16th named storm and the 8th hurricane of the 2010 Atlantic tropical cyclone season. As expected, this year has been very active in the Atlantic basin (Figure 1). Since 1950, there have only been two years that had a greater number of named storms by this point in the season. 1995 had 17 by this date with a seasonal total of 19 and 2005 had 20 with a seasonal total of 27. This season appears very similar to 1995, which also saw the recurvature of every Cape Verde storm prior to reaching the United States.

Considering this season’s heightened activity, the United States has fared remarkably well with Tropical Storm Bonnie the only named storm to officially make landfall over the American coastline. Minor impacts were felt in Texas from Hurricane Alex and Tropical Storm Hermine, which both made landfall over northeastern Mexico. Hurricane Earl and Tropical Storm Nicole passed near the East Coast, but their most severe weather stayed well offshore.

Figure 1: 2010 Tropical Cyclone Tracks

Though the climatological peak of the hurricane season has passed, the United States is not yet out of the woods as the season officially lasts until November 30th. Since 1900, 19 tropical cyclones have made landfall in the United States after this point in the season, 15 of them as hurricanes (Figure 2). 14 of these 19 made landfall along the Gulf Coast of Florida, typically originating over the western Caribbean. The last hurricane to make a U.S. landfall in October was Hurricane Wilma of 2005. It followed a similar path to Hurricane Paula, but moved quickly toward the northeast after passing the Yucatan Peninsula. Wilma made landfall over southwestern Florida as a category 3 hurricane on October 24th and became the 11th most damaging hurricane on record (normalized to 2010 $). based on climatology, the western Caribbean needs to be closely monitored for the remainder of the hurricane season and the Gulf Coast bears the greatest risk.

Figure 2: Tropical cyclones that have made U.S. landfall after October 13th since 190

The current NHC forecast has Hurricane Earl passing about 100 miles east of the Outer Banks and about 75 miles east of Cape Cod, Massachusetts. The barrier islands of North Carolina and the easternmost tip of Massachusetts remain within the cone of uncertainty. Models are in very good agreement regarding Earl’s track through the next 24 hours. Beyond then, model guidance diverges with a couple projecting a New England landfall. A majority of the models are currently projecting that Earl will stay off the coast and not make landfall in the U.S.

The ICAT Damage Estimator can be used to select and display all historical storms that have made landfall since 1900 within the current computer model forecast range for Earl. The westernmost model prediction has Earl passing over the tip of Massachusetts. Accordingly, the ICAT Damage Estimator selects and displays the historical storms that have passed over the easternmost part of Massachusetts. There have been three: Hurricane Edna of 1954, Tropical Storm Esther of 1961, and Tropical Storm Carrie of 1972.

Edna had a similar track and intensity to Earl’s current forecast. It passed east of North Carolina as a category three hurricane and made landfall over Massachusetts as a category one hurricane. It caused about $40M in damage in 1954, which would equate to about $3.67B if it were to make landfall today. Edna is the 65th most damaging storm in the normalized record.

Using the ICAT Damage Estimator’s export feature, you can overlay Edna’s track with the NHC forecast cone for Earl in Google Earth. The output shows that Edna was about 100 miles west of Earl’s current position on September 7, 1954. It tracked up the coast along the western edge of Earl’s forecast cone. Edna passed about 30 miles east of the Outer Banks and made landfall over eastern Massachusetts. Hurricane Earl’s impact should be less severe than Edna’s if the current track forecast verifies. A deviation to the west would lead to a situation more reminiscent of Hurricane Edna.

Tropical cyclones that develop over the eastern Atlantic are known as Cape Verde storms (named after the archipelago off the western coast of Africa). “Cape Verde season” typically begins around August 15th and lasts through September. During this time period, sea surface temperatures are their highest and upper-level winds are their most favorable for tropical cyclone development across the tropical Atlantic.

Only about 20% of Cape Verde storms eventually make U.S. landfall. This is because most of them are steered around the persistent central Atlantic subtropical ridge, known as the Bermuda High. The further south a Cape Verde storm develops, the less likely it is to be steered away by the Bermuda High. The position and strength of the ridge are also very important. A weaker ridge, positioned further east, typically causes a storm to turn N more quickly. A stronger ridge, positioned further west, can steer a storm toward the eastern U.S.

Because these storms form thousands of miles from their ultimate destination, they have plenty of time to grow and intensify. A majority of the most intense and most damaging tropical cyclones in American history were Cape Verde storms. Displaying the ten most damaging U.S. hurricanes with the ICAT Damage Estimator shows that seven of them were Cape Verde storms.

Hurricane Danielle and Tropical Storm Earl are both considered Cape Verde storms. Danielle tracked far enough north for the Bermuda High to steer it away from North America. Living up to its reputation as a Cape Verde storm, Danielle reached category 4 intensity this morning.

Tropical Storm Earl developed further south than Danielle and therefore has a better chance of eventually making landfall. However, Hurricane Danielle has left a trail of relatively low pressure in its wake, which could cause Earl to follow a similar path around the Bermuda High.

The ICAT Damage Estimator shows that there have been ten U.S. landfalling tropical cyclones that have passed within 100 miles of Earl’s current position. Not surprisingly, four of them are in the top ten of the most damaging historical U.S. storms: the Galveston hurricanes of 1900 and 1915, the Lake Okeechobee hurricane of 1928, and the New England hurricane of 1938. Eight of the ten storms eventually impacted North Carolina as they rounded the subtropical ridge about 1000 miles west of where Danielle is currently moving.

By using the export feature in the ICAT Damage Estimator, you can overlay the NHC 5-day forecast cone with these ten storms. The current forecast calls for Earl to begin its northward turn further east than nine of the ten historical storms that ended up impacting the United States. Hurricane Dianne of 1955 (highlighted in pink below) temporarily turned toward the north near Hurricane Danielle’s current position, but was abruptly forced west due to strong ridging to its north. The current upper-air pattern would not likely support similar behavior for Earl. Climatology and the current atmospheric conditions suggest that Earl has a relatively low probability of making U.S. landfall, though impact is not out of the question. As shown by the ICAT Damage Estimator, the greatest threat in the U.S. appears to be around the Carolinas.

Hurricane Alex made landfall June 30, 2010 around 9 PM CDT near the municipality of Soto La Marina, Mexico, which is about 110 miles south of Brownsville, Texas. It made landfall as a category 2 hurricane with sustained winds of 105 mph. Southern Texas experienced wind gusts up to 60 mph and rainfall of 6-12 inches. Alex will weaken quickly as it is now over land. The center of circulation should dissipate within the next 24 hours. The rain and wind will continue to subside in southern Texas. The map below is from the National Hurricane Center, and shows the cumulative wind history of Hurricane Alex with tropical storm force winds in orange and hurricane force winds in red.

Hurricane Alex Cumulative Wind History

Below is a collection of satellite and radar images of Hurricane Alex near landfall. Alex was an abnormally well-organized hurricane for this early in the season. A minimum central pressure of 947 mb was recorded within Alex near landfall. A pressure this low usually corresponds to a category 3 or 4 hurricane. Luckily, the storm made landfall before the winds could catch up with the pressure.

Radar from June 30 - 840 PM CDT

Satellite from June 30 - 545 PM CDT

Satellite from June 30 - 515 PM CDT

When a storm is this close to landfall, it is good to start “nowcasting.” This term is used in the weather world to refer to the act of forecasting based on current observations as opposed to using the various meteorological models available. For the last two days, the models have been indicating that Tropical Storm Alex would eventually turn toward the NW and then the WNW. They’ve also been projecting that Alex would strengthen to a cateegory 1 hurricane prior to landfall. Now that we are less than 36 hours from landfall, it is time to make sure these model projections are panning out by “nowcasting.”

The two visible satellite images below were taken 7 hours apart. The top image is from 645 AM and the bottom image is from 145 PM. The red dots are the official NHC forecast points from this morning. The first point indicates where Alex was at 6 AM. The second point shows where the NHC predicted Alex would be at noon today. The most recent satellite image shows that Alex’s center (green dot) is further south than was predicted this morning. The red line shows the NHC’s forecast and the green line shows reality. By using “nowcasting,” it becomes obvious that Alex has begun its WNW turn. In fact, the NHC has just shifted their track accordingly as I write this. The tropical cyclone also looks better organized in the most recent image, which indicates that Alex is indeed strengthening.

June 29 - 645 AM MDT

June 29 - 145 PM MDT